Police tell the Los Angeles Times that at least 30 shots were fired.
The gunman shot a security guard standing at the front of the bar before entering the building.
Reports say the suspect may have used smoke grenades as well as a gun in the attack.
He could not immediately provide information on the extent of the victims' injuries.
The gunman is described as having a beard and wearing a hat and glasses.
Ventura County Sheriff's Office Capt. Garo Kuredjian said at a news conference early Thursday that a sheriff's deputy was also shot and taken to a hospital.More news: News PUBG PS4 Evidence Continues to Mount
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A police helicopter swirled overhead and officers surrounded the area with cars.
The gunman was still believed to be inside the bar an hour later, according to a police official.
He said responding deputies could hear gunshots as they arrived on scene.
An unnamed witness told the newspaper that someone ran into the bar around 11.30pm (local time) and started shooting what looked to be a black pistol.
Victims rant to a nearby petrol station to get medical help.
A college country night was being held at the venue, which is popular for line dancing.
There is more than 2,500 square feet of dance space.